(1) Field of the Invention
The present invention relates to plethysmographs used for measuring changes in air volume, such as in non-invasive pulmonary testing of small animals, and in particular to plethysmographs in which transducer noise is substantially reduced, improving the accuracy of test data.
(2) Description of the Prior Art
Plethysmographs are used in research to collect data relating to changes in air pressure within a test chamber. An example of such data is pulmonary data from small animals, such as mice. Most plethysmographs are comprised of a test chamber to enclose the test subject, a reference chamber, and a differential pressure transducer connected to the two chambers, e.g., via tubing extending from a port in each chamber to the transducer. Both chambers are in communication with the ambient air, i.e., the air within the room where the tests are being conducted, through restricted airflow openings, or pneumotachs.
As changes to the air volume within the test chamber occur, pressure variations are recorded by the transducer, which normally displays the recorded data in numerical form or as a graph. Air pressure within the test chamber can also vary due to changes in the pressure of air entering the test chamber through the pneumotachs. This non-chamber originated air pressure variation, known as background noise or simply noise, can adversely affect the accuracy of the recorded data, since the transducer also measures the noise.
Plethysmographs are commonly used to measure the pulmonary activities of small animals, such as mice, that are completely or substantially enclosed within the test chamber. As the test animal inhales or exhales, the changes in air volume results in pressure variations that are recorded by the transducer, which normally displays the recorded data in numerical form or as a graph.
The purpose of the reference chamber is to partially reduce the noise affect. A second tube extends from a reference chamber outlet to the transducer. The transducer simultaneously measures variations in air pressures within the two chambers, and subtracts the reference chamber measurements from the animal chamber measurements. As a result, the net pressure variations are essentially attributable to the respiration patterns of the test animal. Preferably, the test and reference chamber pneumotachs are close to each other to minimize variations in exterior air patterns.
A representative plethysmograph of the type used to measure small animal pulmonary responses is shown and described in commonly assigned U.S. Pat. No. 5,379,777 to Lomask, the entire disclosure of the patent being incorporated herein by reference.
While a reference chamber partially addresses the problem of noise in transducer-measured plethysmographs, variations due to air pressure differences still remain. Thus, a level of noise still occurs, which can adversely affect test results, particularly in sensitive measurement. Thus, there is still a need for an apparatus and method for lessening the effect of ambient air changes in plethysmographs used for animal pulmonary measurements, as well as other testing of air volume changes within a test chamber.